Organic electroluminescent device using a mixture of high and low molecular light-emitting substances as a light-emitting substance

ABSTRACT

An organic EL device in which light-emitting efficiency, color purity and laser induced thermal imaging characteristics are improved by providing with an organic EL device comprising a first electrode, a hole transport layer, a light-emitting layer, and a second electrode, wherein the light-emitting layer uses a mixture of an optically active low molecular electric charge transport material and a high molecular light-emitting substance.

CROSS REFERENCE

This application is a continuation of prior U.S. patent application Ser.No. 10/421,754, filed on Apr. 24, 2003 now U.S. Pat. No. 7,052,784,which claims the benefit of Korean Application No. 2002-36558, filed onJun. 28, 2002, both of which are hereby incorporated by reference forall purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescent device,and more particularly, to an organic electroluminescent device using amixture of high and low molecular light-emitting substances as alight-emitting substance so as to enable laser induced thermal imaging(LITI) as a high molecular organic electroluminescent device using ahigh molecular material emitting light under an electric field.

2. Description of Related Art

Generally, an organic electroluminescent device consists of variouslayers including an anode and cathode, a hole injection layer, a holetransport layer, a light-emitting layer, and an electron transportlayer. The organic electroluminescent device is divided into high andlow molecular organic electroluminescent devices depending on a materialto be used, wherein each layer is introduced by vacuum deposition in thecase of the low molecular organic electroluminescent (hereinafterreferred to as EL) device while a light-emitting device is fabricatedusing a spin coating process, or an ink jet process in the case of thehigh molecular organic EL device. In the case of a single color device,an organic EL device using a high molecular material is simplyfabricated using the spin coating process, wherein the organic EL deviceusing a high molecular material has drawbacks of lower efficiency andlife cycle, although it has a lower driving voltage compared to anorganic EL device using a low molecular material. Furthermore, red,green and blue color high molecular materials should be patterned whenfabricating a full color device, and the organic EL device using highmolecular materials has problems in that emitting characteristics suchas efficiency and life cycle are deteriorated when using ink jettechnology or laser induced thermal imaging.

Particularly, in the case of most materials, a single material is nottransferred when patterning the material by using laser induced thermalimaging. A method of forming patterns of an organic EL device by laserinduced thermal imaging is disclosed in Korean Patent No. 1998-51814,and U.S. Pat. Nos. 5,998,085, 6,214,520 and 6,114,088.

In order to apply the heat transfer process, a light source, a transferfilm and a substrate are needed at the least, and light coming out ofthe light source is absorbed by a light absorption layer of the transferfilm to be converted into heat energy so that a transfer layer formingmaterial of the transfer film is transferred onto the substrate by heatenergy to form a desired image, as disclosed in U.S. Pat. Nos.5,220,348, 5,256,506, 5,278,023 and 5,308,737.

The heat transfer process can be used to fabricate a color filter forliquid crystal display devices, or to form patterns of a light-emittingsubstance, as disclosed in U.S. Pat. No. 5,998,085. Although it iswritten in U.S. Pat. No. 5,998,085 that a light-emitting substance foran organic EL device is transferred onto a substrate, characteristics onmaterials used to improve transfer properties are not mentioned in thepatent.

Furthermore, although there are such patents as U.S. Pat. No. 6,117,567for creating other colors using phase separation of light-emittingsubstances, Korean Patent No. 2001-3986 for increasing efficiency, orU.S. Pat. No. 5,965,281 for improving device characteristics by addingionic surfactants, all of the patents are related to improvement ofcharacteristics of materials themselves. Therefore, literature andpatents regarding improvement of high molecular materials duringpatterning using laser induced thermal imaging do not exist currently.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of the present invention to provide anorganic EL device using a mixture of high and low molecularlight-emitting substances as a light-emitting substance in which a highmolecular light-emitting layer can be patterned, and color purity andlight-emitting characteristics are improved when fabricating a fullcolor high molecular organic EL device by laser induced thermal imaging.

Additional aspects and advantages of the invention will be set forth inpart in the description which follows and, in part, will be obvious fromthe description, or may be learned by practice of the invention.

The foregoing and/or other aspects of the present invention are achievedby providing an organic EL device comprising a first electrode, a holetransport layer, a light-emitting layer and a second electrode, whereinthe light-emitting layer uses a mixture of an optically active lowmolecular electric charge transport material and a high molecularlight-emitting substance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a drawing illustrating a transfer mechanism when transferpatterning an organic EL layer used in an organic EL device using alaser; and

FIG. 2 is a graph illustrating color purity improvement results of agreen light-emitting substance as a spectrum of photoluminescence andelectroluminescence of a low molecular material (CBP), a light-emittinghigh molecular material and a mixture thereof.

FIG. 3 is a cross-sectional view illustrating an organic EL displaydevice according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

As illustrated in FIG. 1, an organic layer S2, which is adhered onto asubstrate S1, must be separated from a part of S1 where the laser is notreceived as the organic layer S2 is separated from the substrate S1 andtransferred to a substrate S3 by action of the laser in a mechanism oftransfer patterning of an organic layer using an ordinary laser.

FIG. 3 shows a cross-sectional view illustrating an organic EL displaydevice incorporating an organic EL layer according to the presentinvention. In FIG. 3, reference numerals 100, 200, 300 and 400 denote acathode, a light-emitting layer, a hole transporting layer, and ananode, respectively.

Factors influencing transfer characteristics are adhesion force (W12)between the substrate S1 and the film S2, adhesive force (W22) betweenthe films, and adhesion force (W23) between the film S2 and thesubstrate S3. These adhesion and adhesive forces are represented assurface tensions (y1,y2,y3) and interfacial tensions (y12,y23) of eachlayer as in the following expressions:W12=y1+y2−y12;W22=2y2; andW23=y2+y3−y23.

In order to improve laser induced thermal imaging characteristics, theadhesive force between the films should be less than the adhesion forcebetween each substrate and the film. Generally, a high molecular film isused as a light-emitting substance including a light-emitting layer inan organic EL device, wherein the high molecular film may not have goodtransfer characteristics when patterning using a laser since it has ahigh adhesive force between films as a material having a high molecularweight.

Therefore, transfer characteristics can be improved either by loweringthe adhesive force between the films or by increasing the adhesion forcebetween the film and a substrate.

In the case of a high molecular light-emitting substance including alight-emitting layer of the present invention, it is difficult tomanufacture light-emitting layer patterns by laser induced thermalimaging since the adhesive force between high molecular light-emittingsubstances themselves is very high compared with the adhesion forcebetween the light-emitting substance and a substrate, or the adhesionforce between the light-emitting substance and the surface of a donorfilm on which the light-emitting layer is coated. In order to solvethese and other problems, the high molecular material is used togetherwith a low molecular material having a relatively low adhesive forcebetween the films, and a high molecular matrix preventing phaseseparation between a high molecular material and a low molecularmaterial and helping formation of film is additionally added if it isnecessary. A material inhibiting phase separation and functioning as abinder, which are generally used as a high molecular matrix improvingcoating uniformity of an organic layer, preferably is an optically inerthigh molecular material selected from a group consisting of polystyrene,poly(4-methylstyrene), poly(α-methylstyrene), polymethylmetacrylate(PMMA), polyethylmetacrylate, poly(vinyl pyridene), poly(vinylpyridine), polyphenyleneoxide (PPO), styrene-butadiene block copolymer,styrene-metacrylic acid ester copolymer, styrene-methylmetacrylatecopolymer, polycarbonate, polyethyleneterephthalate, polyestersulfonate,polysulfonate, polyacrylate, polyimide fluoride, transparentfluorocarbon resin, and transparent acryl based resin. The highmolecular matrix preferably has a weight ratio of 0 or more, or 0.9 orless for the total weight of the light-emitting layer. “Optically inert”means that the final light-emitting spectrum and color coordinates inthe visible light range (450˜800 nm) showing a light-emitting substanceare not influenced even though additives are introduced.

On the other hand, all of the materials having a structure includingbasically used light-emitting high molecular materials such aspolyfluorene, polyspiro and poly(vinylene phenylene) can be used as ahigh molecular light-emitting substance contained in the light-emittinglayer. Furthermore, in the case of using a high molecular matrix,generally used low molecular light-emitting substances such as fluorene,phenylene, anthracene, etc., can be further added to the high molecularlight-emitting substances as light-emitting substances.

Furthermore, materials that have electric charge transport capabilitiesand are optically active including a low molecular hole transportmaterial used as a host substance of an electrophosphorescent device, anamorphous hole transport material having a high glass transitiontemperature and an electron transport material, are used as a lowmolecular light-emitting substance. “Optically active materials” meanmaterials showing photoluminescence characteristics having a peak in therange of 350˜650 nm. Carbazole based 4,4-N,N′-dicarbazole-biphenyl (CBP;photoluminescence peak, λmax=377 nm) or arylamine basedN,N′-8-bis-1-naphthylyl-diphenyl-1,1′-biphenyl4,4′-diamine (α-NPB;λmax=433 nm) having hole transport capability is preferably used as alow molecular electric charge transport material. Oxadiazole basedmaterial, preferably2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,2,4-oxadiazole (PBD; λmax=439nm) is used as a material having electron transport capability.Furthermore, starburst amine based4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA; λmax=390 nm),4,4′,4″-tris(N-3-methyl phenyl amino)-triphenylamine (m-MTDATA; λmax=428nm) and 1,3,5-tris-(N,N-bis-(4-methoxy-phenyl)-aminophenyl)-benzene(TDAPB; λmax=439 nm) can also be used as a material having electrontransport capability.

An amount of low molecular light-emitting substances to be usedpreferably has a weight ratio of 0.1 or more, or 0.9 or less for thetotal weight of the light-emitting layer.

The mixing weight ratio of the light-emitting layer can be controlleddepending on color purity, efficiency and patterning characteristics ofa device. A fabrication method of a high molecular organic EL deviceaccording to an embodiment of the present invention is as follows.

The air blow treated substrate passes through a supersonic cleaningprocess using a neutral detergent, acetone, isopropyl alcohol (IPA),etc., after air blow treating a patterned ITO substrate. A highmolecular layer used as a hole injection layer is spin coated on the ITOsubstrate after removing moisture and an organic material pollutionsource by treating UV/O₃ onto the surface of the cleaned and dried ITOsubstrate for more than 15 minutes, and then the high molecular layerspin coated on the ITO substrate is baked at a high temperature toremove residual moisture. In the case of fabricating a single colordevice, the device is completed by encapsulating the deposited layerafter spin coating a mixed layer to a thickness of tens of nm andcathode depositing the spin coated mixed layer.

A fabrication method of a light-emitting layer patterned devicecomprises the processes of laying up a hole injection layer and atransport layer on a substrate by spin coating, transferring a highmolecular mixed light-emitting layer spin coated on a donor film to athickness of tens of nm on ITO patterns by laser induced thermalimaging, cathode depositing the high molecular mixed light-emittinglayer transferred ITO patterns, and encapsulating the depositedmaterial, thereby finally completing the device.

An edge roughness of a light-emitting layer of the above-fabricatedorganic EL device can be maintained to less than 8 μm.

EXAMPLES

The present invention will be explained in more detail with reference toExamples hereinafter.

Examples 1 and 2

A low molecular hole transport material, 4,4′-N,N′-dicarbazole-biphenyl(CBP) manufactured by Universal Display Corporation, was dissolved intotoluene in a concentration ranging from 1.0 to 2.0 wt %. As a highmolecular matrix, polystyrene having a molecular weight of 50,000manufactured by POLYSCIENCE CORPORATION and poly(4-methylstyrene) havinga molecular weight of 70,000 manufactured by ALDRICH CORPORATION weredissolved into toluene in a concentration ranging from 1.0 to 2.0 wt. %respectively. As a high molecular light-emitting substance, Green K2manufactured by DOW CHEMICAL CORPORATION, which is a polyfluorene basedgreen light-emitting substance, was dissolved into toluene in aconcentration ranging from 1.0 to 2.0 wt. %. The agitated solutions weremixed in an appropriate mixing ratio after completely dissolving each ofthe solutions by sufficiently agitating each of the solutions at atemperature of 60° C. for more than 3 hours. The mixed solution film wasstored in a nitrogen atmosphere after agitating the mixed solution at anordinary temperature for more than 1 hour, and forming a mixed solutionfilm having a thickness of 50 to 80 nm by spin coating the mixedsolution on a transfer donor film in the atmosphere. PEDOT/PSS having amodel name of CH8000 manufactured by BAYER AG was spin coated on theUV-O₃ treated substrate in the atmosphere to form a hole injection layerafter UV-O₃ treating the ultrasonic cleaned substrate for 15 minutesafter ultrasonic cleaning an anode patterned ITO substrate. A substratefor laser induced thermal imaging was manufactured by spin coating asolution as a hole transport layer and a primary layer that is preparedby dissolving BFE manufactured by DOW CHEMICAL CORPORATION also intotoluene in a concentration of 0.4 wt. % to a thickness of 10 to 30 nm onthe moisture removed hole injection layer after removing residualmoisture in the PEDOT layer by baking the hole injection layer at a hightemperature of 100° C. or more for several minutes.

An organic layer coated transfer film was covered on the substrate andthermally transferred onto the substrate by using a laser. A device wasfabricated by sequentially depositing 2 nm of LiF and 300 nm of Al asthe cathode on the heat treated light-emitting layer and encapsulatingthe cathode deposited light-emitting layer with a glass substrate afterheat treating a patterned light-emitting layer in a nitrogen atmosphereat a temperature of 130° C. for one hour. It is not possible to formpatterns using laser induced thermal imaging if only green K2manufactured by DOW CHEMICAL CORPORATION is used as a raw material.Weight ratio ranges of CBP and polystyrene for the total mixedlight-emitting layer in which laser transfer was possible and efficiencywas satisfactory were 0.25≦CBP≦0.5 and 0≦polystrene≦0.5. An edgeroughness of the transferred film was 5 to 8 μm. Characteristics of alaser induced thermal imaging organic EL device fabricated using alight-emitting layer in which green K2/CBP/polystyrene and greenK2/CBP/poly(4-methylstyrene) were respectively mixed in a weight ratioof 1:1:1 is represented in the following Table 1:

TABLE 1 Structure of device: ITO/PEDOT (60 nm)/BEF (20 nm)/EML (50-90nm)/LiF (2 nm)/Al (250 nm) Efficiency Driving Voltage EML (Cd/A) (500Cd/m2) CIE x CIE y EXAMPLE 1 Green K2/CBP/polystyrene 8.0 4.1 0.36 0.60(1:1:1) EXAMPLE 2 Green K2/CBP/poly(4- 4.2 4.7 0.36 0.60 methylstyrene)(1:1:1)

Examples 3, 4 and Comparative Examples

In examples 3 and 4, an organic EL device was fabricated by forming alight-emitting layer by spin coating after respectively dissolving greenK2, a high molecular material, CBP, polystyrene, poly(4-methylstyrene),etc., into toluene in a concentration ranging from 1.0 to 2.0 wt. % sothat they have the same weight fractions as the high molecularlight-emitting substances used in the Examples 1 and 2. The mixedsolution was used in a spin coating process after agitating each of thesolutions prepared by dissolving green K2, a high molecular material,CBP, polystyrene, poly(4-methylstyrene), etc., into toluene at atemperature of 60° C. for more than 3 hours, and mixing each of theagitated solutions to a certain mixing ratio.

PEDOT/PSS having a model name of CH8000 manufactured by BAYER AG wasspin coated on the UV-O₃ treated substrate in the atmosphere to form ahole injection layer after UV-O₃ treating the ultrasonic cleanedsubstrate for 15 minutes after ultrasonic cleaning an ITO substrate. Themixed solution film was heat treated at a temperature of 130° C. in anitrogen atmosphere for one hour after removing residual moisture in thePEDOT layer by baking the hole injection layer at a high temperature of100° C. or more for about several minutes, and forming a mixed solutionlayer having a thickness of 50 to 80 nm by spin coating a light-emittinglayer that is dissolved by toluene on the moisture removed holeinjection layer. A device was fabricated by sequentially depositing 2 nmof LiF and 300 nm of Al as the cathode on the light-emitting layer andencapsulating the cathode deposited light-emitting layer with a glasssubstrate. Characteristics of an organic EL device fabricated by spincoating a light-emitting layer in which green K2/CBP/polystyrene andgreen K2/CBP/poly(4-methylstyrene) were respectively mixed in a weightratio of 1:1:1 is represented in the following Table 2. The performanceof the organic EL device was compared with that of an organic EL devicefabricated by spin coating green K2 of a high molecular material only.An organic EL device using a pure green K2 light-emitting layer can notbe patterned by laser induced thermal imaging while an organic ELdevice, using a mixed light-emitting layer of a low molecular materialand polystyrene, can be patterned by laser induced thermal imaging,wherein results of improved efficiency and color coordinates at 500Cd/m² are represented in Table 2. Here, a light-emitting layer of greenK2/CBP/polystyrene (1:1:1) had 11.2 Cd/m² (8.5 lm/N) and colorcoordinates of 0.35 and 0.60, wherein efficiency was 500 Cd/m² at CIE1931 and 5V.

TABLE 2 Structure of the device: ITO/PEDOT (60 nm)/EML (50-90 nm)/LiF (2nm)/Al (250 nm). Efficiency (Cd/A) EML at 500 nit CIE x CIE y EXAMPLE 3Green K2/CBP/polystyrene 11.2 0.35 0.60 (1:1:1) EXAMPLE 4 GreenK2/CBP/poly(4- 8.6 0.36 0.60 methylstyrene) (1:1:1) COMPARATIVE Green K27.6 0.40 0.60 EXAMPLE

Examples 5 and 6

Measuring the results of coordinates for a light-emitting diodefabricated by blending a low molecular hole transport material into ahigh molecular light-emitting substance are shown in the Examples 5 and6. Each of the solutions were mixed in an appropriate weight ratio andagitated at an ordinary temperature for more than one hour after mixing4,4′-N,N′-dicarbazole-biphenyl (CBP; manufactured by Universal DisplayCorporation), a green high molecular light-emitting substance having abrand name of GREEN manufactured by COVION CORPORATION and a blue highmolecular light-emitting substance having a brand name BLUE Jmanufactured by DOW CHEMICAL CORPORATION with toluene in a concentrationranging from 1.0 to 2.0%, and completely dissolving the raw materialmaterials in toluene by sufficiently agitating the mixture in toluene ata temperature of 60° C. for more than 3 hours. An organic EL device wasfabricated by forming a light-emitting layer also in a spin coatingprocess. As in the Examples 3 and 4, PEDOT/PSS having a model name ofCH8000 manufactured by BAYER AG was spin coated on the UV-O₃ treatedsubstrate in the atmosphere to form a hole injection layer after UV-O₃treating the ultrasonic cleaned substrate for 15 minutes afterultrasonic cleaning an ITO substrate. A mixed solution layer was formedby spin coating a light-emitting layer that is dissolved by toluene onthe moisture removed hole injection layer after removing residualmoisture in the PEDOT layer by baking the hole injection layer at a hightemperature of 100° C. or more for about 5 minutes. A device wasfabricated by sequentially depositing 2 nm of LiF and 300 nm of Al asthe cathode on the light-emitting layer and encapsulating the cathodedeposited light-emitting layer with a glass substrate.

Characteristics of an organic EL device fabricated by spin coating alight-emitting layer, in which GREEN manufactured by COVION CORPORATIONthat is a polyvinyl phenylene based green light-emitting high molecularmaterial/CBP and BLUE J manufactured by DOW CHEMICAL CORPORATION that ispolyfluorene blue high molecular material/CBP were respectively mixed ina weight ratio of 1:3, are represented in the following Table 3.Improved effects of color coordinates according to the present inventioncould be confirmed since CIE color coordinates were (0.35, 0.59) and(0.15, 0.19) respectively in the case of fabricating an organic ELdevice by spin coating using high molecular materials such as GREENmanufactured by COVION CORPORATION and BLUE J manufactured by DOWCHEMICAL CORPORATION only. In the case of mixed material compositions ofthe Examples 5 and 6, an edge roughness of less than 8 μm was possibleby laser induced thermal imaging.

TABLE 3 Structure of the device: ITO/PEDOT (60 nm)/EML (50-90 nm)/LiF (2nm)/Al (250 nm) EML Efficiency (Cd/A) CIE x CIE y EXAMPLE 5 Greenmanufactured 3.10 (500 nit) 0.27 0.59 by Covion Corporation/CBP (1:3)EXAMPLE 6 Blue J/CBP (1:3) 1.62 (150 nit) 0.15 0.14

It can be seen that a color purity of the mixture of a low molecularcharge transport material and a green light-emitting high molecularmaterial was improved from FIG. 2, which are photoluminescence andelectroluminescence spectrums of a low molecular material (CBP), a greenlight-emitting high molecular material (Green of Covion Corporation),and a mixture thereof (Example 5).

As described above, a mixed light-emitting layer was fabricated in thepresent invention in which the amount of an optically inert polymer wasreduced or removed by adding a low molecular hole transport materialwidely used as a host substance of a phosphorescent device to a highmolecular light-emitting substance. Accordingly, laser induced thermalimaging characteristics were superior by having an edge roughness of 5to 8 μm, and 50% or more of efficiency improvement could be obtainedhaving an efficiency of 11.2 Cd/A (7.6 Cd/A in the case of a pure highmolecular material) under the same luminescence condition (500 Cd/m²).Color purities of green and blue devices were improved by variation of alight-emitting spectrum due to mixing of the low molecular material thatis optically active.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A method for fabricating an organic EL device comprising: providing asubstrate; forming a first electrode on the substrate; forming a holeinjection layer over the substrate; forming a hole transport layer overthe hole injection layer; forming a light-emitting layer on the holetransport layer by Laser Induced Thermal Imaging (LITI) method; andforming a second electrode over the light-emitting layer, wherein thelight-emitting layer uses a mixture of an optically active low molecularelectric charge transport material, a high molecular light-emittingsubstance, and a high molecular matrix and the light-emitting layer ispatterned to an edge roughness of 5 to 8 μm, wherein the hole injectionlayer is spin coated on the substrate after treating UV/O3 onto asurface of the substrate, and wherein the low molecular electric chargetransport material is 4,4′-N,N′-dicarbazole-biphenyl (CBP), and has aweight ratio between 0.1 and 0.9 of the total weight of thelight-emitting layer, wherein the optically active low molecularelectric charge transport material is dissolved in toluene in aconcentration ranging from 1.0 to 2.0 wt %, the high molecular matrix isdissolved in toluene in a concentration ranging from 1.0 to 2.0 wt %,and the high molecular light-emitting substance is dissolved in toluenein a concentration ranging from 1.0 to 2.0 wt %.
 2. The method accordingto claim 1, wherein the high molecular light-emitting substance is amaterial selected from polyfluorene (PFO), polyspiro and PPV (polyphenylene vinylene).
 3. The method according to claim 1, wherein thehigh molecular matrix is an optically inert molecular material selectedfrom a group consisting of polystyrene, poly (4-methylstyrene), poly(α-methylstyrene), polymethylmetacrylate (PMMA), polyethylmetacrylate,poly (vinyl pyridene), poly (vibyl pyridine), polyphenyleneoxide (PPO),styrene-butadiene block copolymer, polycarbonate,polyethyleneterephthalate, polyestersulfonate, polysulfonate,polyacrylate, polyimide fluoride, transparent fluorocarbon resin, andtransparent acryl based resin.
 4. The method according to claim 1,wherein the high molecular matrix has a weight ratio of 0 or more, or0.9 or less for the total weight of the light-emitting layer.
 5. Themethod according to claim 1, wherein the dissolved optically active lowmolecular electric charge transport material, the dissolved highmolecular matrix, and the dissolved high molecular light-emittingsubstance are each agitated at a temperature of 60° C. for more than 3hours and are mixed to form a mixed solution.
 6. The method of claim 5,wherein the mixed solution is stored in a nitrogen (N) atmosphere formore than 1 hour at room temperature and the mixed solution is used toform a mixed solution film having a thickness of 50 to 80 nm on atransfer donor film by spin coating the mixed solution.